Due to its known role in protein turnover in the cell, the ubiquitin-proteasome pathway is expected to be a key player in cellular aging. The proteasome, a multifunctional, multisubunit proteolytic enzyme, is a focal point in the pathway and is in charge of controlled cleavage of short-lived metabolic regulator proteins and damaged long-lived proteins. Both the regulatory role and the "garbage removal" function may have a role in age-related metabolic slowdown and diseases. However, studies implicating the proteasome in aging have been limited, inconclusive, and at times contradictory. I propose to characterize proteasomal assemblies in aging mice liver tissue in a truly comprehensive manner: separately in distinct subcellular compartments, and taking into account activity, specificity, assembly status, and subunit composition. The emerging evidence that proteasomes perform specific duties based on subcellular location justifies my approach. I hypothesize that there is a set of age-related alternations, which collectively form a characteristic pattern of properties that constitutes a signature of aging. I wish to call such signature the proteasome aging index. My preliminary data support the notion that such index can be revealed. Specifically, I propose to: (1) define the proteasome aging index in each age group specific for subcellular localization of proteasomes. I will perform molecular profiling of the following properties of proteasome: content and composition of proteasome subassemblies, protein content of selected subunits of the subassemblies and peptidase activities of all three active sites, determined with model substrates. (2) I will correlate the proteasome index with the pattern of cleavage of natural substrates. (3) Finally, I will employ the proteasome aging index to establish the state of the proteasome pathway in the oxidative stress mice models: the SOD1-/- mice characterized by an increased oxidative stress and mice subjected to caloric restriction accepted as a model of decreased oxidative stress. Successful launching of the proteasome aging index will serve as a novel platform to pursue age-related alterations in the proteasome and its molecular context, learning about physiological consequences of the changes, and propose the means to correct them. The proteasome, a multifunctional enzyme responsible for controlled degradation of most of cellular proteins, has been linked to an age-related decline in protein turnover. I plan to define a pattern of age-related changes in the proteasome: the "proteasome index of aging". Such an index will help to detect signs of cellular aging and help to establish treatments aiming at controlling the aging processes.